First of all, I like this piece! It has kind of an epic feel to it, especially with the addition of the vocal choirs. You could have held the last note a little longer though, and also developed the organ part more, which seemed a little random. I like the contrast of the solo violin with the big choir. This piece kind of reminded me of soundtrack music, because it was really epic, but not too melodic. Perhaps you were looking for this kind of sound, and that's ok for a short piece like this, but you should think more about memorable melodies if you want to engage the listener for longer.
Also, as to how it relates to Thanatopsis, I might have written a quieter piece to make it seem more reflective, but I still got the feeling of death from this one. The regal nature of this piece sort of reminded me of the author's point in that poem that from the simplest peasant to the greatest king, we all ultimately suffer the same fate; it also reminded me of the author's mention of the majesty of nature (hey, maybe you can use that in your presentation:P ).
Overall, I enjoyed this very much.
P.S. For 2 and a half hours of work, this is very good.